An igniter plug used in a gas turbine engine, a diesel engine, a burner igniter, etc., generally includes a center electrode, an insulator disposed externally of the center electrode, and a ground electrode (also called an “outer electrode”) provided externally of the insulator. A forward end portion of the ground electrode; i.e., a ground electrode forward-end portion, forms a gap for discharge in cooperation with the center electrode. Herein, a side of the igniter plug toward the gap is called the “forward side,” and a side opposite the forward side is called the “rear side.”
In a conventional igniter plug, the insulator is fixed in the ground electrode such that its forward end surface (hereinafter, called the “insulator forward-end surface”) is in contact with a surface of the ground electrode (hereinafter, called the “ground electrode counter surface”) which faces the insulator forward-end surface in the axial direction.
In such an igniter plug, by use of cooling fluid (e.g., air) which flows in through inlets provided in a side wall of the ground electrode, the insulator, the ground electrode forward-end portion, and the center electrode are cooled. For example, cooling fluid flows through flow paths which extend from the inlets to outlets provided in the ground electrode forward-end portion. The outlets are disposed radially outward of the inner circumference of the ground electrode forward-end portion. The cooling fluid flows along an annular space formed between the insulator and the ground electrode, thereby cooling the insulator and the ground electrode forward-end portion (See, for example,) Japanese Patent Application Laid-Open (kokai) No. S59-040481.
In some cases, when the conventional igniter plug is heated to a high temperature, because of difference in coefficient of linear expansion between the insulator and the ground electrode, a gap is generated between the insulator forward-end surface and the ground electrode counter surface which are in contact with each other at room temperature. Also, in some cases, when a gap is generated between the insulator forward-end surface and the ground electrode counter surface, cooling fluid which flows through the cooling-fluid paths enters the gap and rapidly cools the insulator, causing cracking in the insulator by thermal shock (heat drop).
The present invention has been conceived to solve the above-mentioned conventional problem, and an object of the invention is to restrain cracking of an insulator of an igniter plug.